1. Field of the Invention
The present invention generally relates to electrical transformers and more particularly to electrical transformers equipped with means having particular structure and geometry for obtaining low noise levels.
It is well known that electrical transformers generally comprise a tank containing a transformer fluid (i.e. mineral oil), a transformer core and a winding subassembly. The transformer core and winding subassembly are generally placed in the transformer fluid and are spaced apart from the tank of the transformer.
2. Discussion of the Background
It is also well known that noise from transformers is a problem for utility industries or companies, especially when the transformer is installed in urban areas.
The people skilled in the art know that noise in transformers is generated by vibration of the core and winding subassembly during electromagnetic operation and by cooling ventilators used for extracting heat, generated during electromagnetic operation, from the tank to the surrounding atmosphere. While noise from cooling ventilators can be effectively reduced by designing lower speed and larger diameter ventilators equipped with low noise blades, the reduction of the noise from the core and winding subassembly vibration is still a problem, given the fact that solutions known in the state of the art are affected by several drawbacks.
Most of the conventional approaches of the state of the art are oriented to consider passive solutions, for reducing the core noise.
A known approach is to add mass to the transformer core in order to avoid core vibration. Unfortunately, this approach leads to transformers having quite larger core sectional area, with significant increase of weight and costs.
Other approaches consider the use of passive devices inside the transformer tank. These devices are constituted by stacks of layers that are made of materials having elastic properties (i.e. rubber). Being placed inside the transformer tank, they act as damping elements adsorbing transformer fluid pressure waves generated by the core and winding subassembly vibration. The main drawback of this approach is due to the fact that these devices can be designed only for adsorbing fluid pressure waves having a certain amplitude and frequency. If a variation of the frequency and the amplitude of such pressure waves occurs, for example due to changed operational conditions of the transformer, the damping action of these devices may not be effective.
The use of active devices for noise reduction is disclosed, for example, in the U.S. Pat. No. 5,726,617.
In the mentioned patent, the use of dynamic-pressure varying devices, placed inside the transformer tank, is considered. Said means, constituted for example by hydraulic actuators or pumps or other similar devices, vary dynamically the pressure of the transformer fluid in order to reduce the pressure waves generated by the operating core and winding subassembly. An active damping device, placed between the transformer tank and the transformer core and winding subassembly is also disclosed. Its function is to damp actively the vibrations of the core and winding subassembly. Vibration sensors and a controller of the mentioned active devices are placed inside or outside the tank.
The solution described in the mentioned patent, appears, however, of difficult implementation, given the fact that no particular attention is provided to the structure of the mentioned active devices. Actually, a significant amount of energy is required for actuating hydraulic actuators or pumps or similar devices. Moreover due to the not negligible size of such devices, only a small number of them can be placed inside the tank. This fact implies a noise cancellation, which is certainly not optimal, being dependent on the particular position of the dynamic-pressure varying devices. In addition, complicated assembling operations are evidently required for mounting said hydraulic actuators and/or similar devices. Complicated assembling operations are also required for mounting said damping devices placed between the core and the tank of the transformer. This fact further increases the manufacturing costs.
It is an object of the present invention to provide an electrical transformer able to overcome the above mentioned problems, in particular without having any significant increase of costs for manufacturing or assembling devices able to cancel the noise from core and winding subassembly.
A further object of the present invention is to provide an electrical transformer which uses, for reducing the noise generated by the core and winding subassembly, a plurality of active devices that are able to vary the transformer fluid volume inside the transformer tank and are very simple to be placed inside the inner surface of the tank.
An other object of the present invention is to provide an electrical transformer comprising a plurality of active devices, which can be easily controlled depending on the operating conditions of the transformer.
In order to achieve these objects and others that will become apparent hereinafter, it is provided an electrical transformer, according to the present invention, which comprises:
a tank containing transformer fluid;
a transformer core and winding subassembly disposed in said transformer fluid within and spaced apart from said tank;
active means for varying the volume of said transformer fluid in order to reduce pressure waves generated by the vibration of said core and winding subassembly during electromagnetic operation, said active means being disposed in said transformer fluid within said tank.
The transformer, according to the present invention, is characterised in that said active means comprise at least a cell having:
a main body and a corrugated membrane operatively connected to said main body in order to realise a sealed container able to maintain a low pressure atmosphere inside;
actuating means placed inside said sealed container and solidly connected to said corrugated membrane.